This invention relates generally to temperature control apparatus for extruders and more particularly to air cooling apparatus for vertical extruders.
Air cooling apparatus is generally known for horizontal extruders which have a heat-exchange jacket surrounding the extrusion cylinder. The heat-exchange jacket contains electrical heating units internally and external ring-like heat-exchange fins. Hoods are located around the fins providing passages between the fins. The hoods form a lower opening beneath the fins and an upper opening above the fins. Air is forced into the lower or inlet opening by a blower and travels past the fins via the passageways between the fins and out through the upper or outlet opening. The fins are cooled by the passing air which in turn reduces the temperature of the heat-exchange jacket.
Electrical control means, including temperature probes, measure the temperature of the extruder barrel and are effective to either actuate the electrical heating units in the heat-exchange jacket for heating or to turn on the blower for cooling. In this manner, the barrel temperature can be maintained at a desired set temperature.
An example of a temperature control apparatus for a horizontal extruder of the type described above is shown in U.S. Pat. No. 3,353,212 dated Nov. 21, 1967 in which I am a co-inventor. Although the cooling apparatus in this patent functions satisfactorily for horizontal extruders it is inadequate when applied to vertical extruders. In a horizontal extruder, the air flow is vertical due to the fact that the hood openings are located above and below the fins. The natural tendency of hot air to rise is a factor in the air flow between the lower and upper openings in the hood, particularly in maintaining an even air flow throughout the entire fin area. This natural upward air flow is even more significant when the blower is off. As the heated air from between the fins rises, cooler air will be drawn from the lower opening and will cause additional cooling of the fins.
The above described air-cooling apparatus has been found to be unsatisfactory when it is applied to a vertical extruder. Since the longitudinal axis of a vertical extruder is vertical, the opening of the hood will be aligned within a horizontal plane and the air flow from the blower or inlet opening to the opposite or outlet opening is also in this horizontal plane. The natural upward flow of air creates an uneven air flow between the fins which in turn creates unequal cooling of the fins.